USB: xhci: Set multi-TT field for LS/FS devices under hubs.

When setting up a slot context for an address device command, set the
multi-TT field if this is a low or full speed device under a HS hub with
multiple transaction translators.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>